1. Field of the Invention
The present invention relates to a thermal transfer ribbon and a method of manufacturing the same, and more particularly, to a thermal transfer ribbon having a good anti-static property and capable of forming printed letters or images excellent in anti-abrasion property and image quality.
2. Description of the Related Art
Conventionally, a fused-ink transfer system has been widely known as a printing method in which a thermal transfer ribbon is used. The thermal transfer ribbon is manufactured by forming a heat-fusible color layer onto a substrate sheet such as plastic film or the like. The heat-fusible color layer is formed by dispersing a color material such as pigment, dye or the like into a binder such as heat-fusible wax, resin or the like. In the above transfer system, an energy of an amount corresponding to image information is applied to the thermal transfer ribbon by a heating means such as thermal head or the like so that the color material together with the binder are transferred to an image-receiving sheet such as paper, plastic film or the like.
Thus printed image formed in accordance with the fused-ink transfer system has a high density and is excellent in sharpness, so that the fused-ink transfer system is suitable for recording binary images such as letter, line-drawing or the like. In addition, when a thermal transfer ribbon comprising color layers containing yellow, magenta, cyan, black dyes or the like is used and an image is printed and recorded on the image-receiving sheet so that the respective color layers are superposed, it becomes also possible to form a multi-color image or full-color image through a subtractive color process.
Such the printing method using the thermal transfer ribbon for the fusible-ink transfer system has been widely applied to various fields. For example, the image information is printed on the transfer-receiving materials such as ordinary paper (plain paper), coat paper, plastic sheet or the like by means of various thermal transfer printers such as label printer, bar-code printer, facsimile printer, word-processor printer or the like.
In view of workability at the time of replacement of the thermal transfer ribbon in the printer and in order to prevent the thermal head from being broken by dust adhesion in the printer, a requirement for a thermal transfer ribbon having anti-static function has been gradually increased in these days.
The thermal transfer ribbon providing blackish (black color) image with the carbon black easily can impart an electrical conductivity to the thermal transfer ribbon because of a grain structure of the carbon black. In general, when the carbon black is contained in one continuous layer at an amount of 30 weight % or more, the electrical conductivity can be secured in most cases. Therefore, a problem of anti-static function would be seldom raised.
Japanese Patent Laid-Open Publication No. HEI 2-63791 as a prior art discloses a thermal transfer ribbon in which Ketchen black as the electrically conductive carbon black is contained in a heat-fusible ink layer thereby to impart anti-static property to the thermal transfer ribbon.
In the embodiments disclosed in this Japanese Patent Publication No. HEI 2-63791, a thermal transfer ribbon mainly composed of wax is used. However, in a case where the color layer contains a large amount of the wax, the printed image is inferior in durability such as anti-abrasion property, alcohol resistance or the like.
On the other hand, in a case where the color layer mainly composed of thermoplastic resin as a binder structurally containing a lot of amorphous portions is provided in order to improve durability of the printed image, an apparent volume into which carbon black is dispersed will become large, and an amount of the electrically conductive carbon black required for securing the anti-static effect is disadvantageously increased. As a result, there were posed problems such that a dispersibility of carbon black is deteriorated, so that a coating liquid containing carbon black cannot be actually available. Otherwise, a stability of ink cannot be maintained for a long time, so that agglomerates of carbon black are liable to settle into the ink.
Further, according to the embodiments disclosed in the Japanese Patent Laid-Open Publication No. HEI 2-63791, a thickness of the ink layer was relatively large, so that the above prior art was not a technique capable of realizing a thin-film ribbon having a printing sensitivity enough to be applied to a high-speed type printer. In addition, in a case where a layer for improving the function of the ribbon is provided on the color layer, the above prior art was not a technique capable of sufficiently lowering a surface resistance value of the ribbon.
The other prior art, Japanese Patent Laid-Open Publication No. HEI 8-300830 discloses a thermal transfer recording medium in which a hot-melt ink layer is provided on a base material sheet and the hot-melt ink layer contains carbon black with a tinting strength index of 125 or less. The thermal transfer recording medium in this publication is good in running properties during printing and capable of obtaining a transfer image having a sufficient black degree even in low energy printing. However, this publication describes that a preferable amount of the carbon black contained in 100 parts by weight of the hot-melt ink layer is 30 parts by weight or more.
Furthermore, in a case where the color layer is formed by a coating liquid prepared by dispersing carbon black into a water-type resin, a grain structure of carbon black is easily maintained, so that the electrical conductivity of the ribbon can be easily secured. In this case, however, the binder resins in a form of emulsion particles aggregate to each other to form a discontinuous film in a micro scale. As a result, the anti-static effect is increased, but there are posed disadvantages such that the durability, in particular, a resistance to alcohol is lowered and the blocking is liable to occur under high temperature and high humidity conditions.
In recent years, there has been a strong demand for increasing sensitivity of printing media in accordance with the high-speed type printer and a demand for improving performance of the printed matter. However, the situation in these days makes it difficult to improve the performance of the thermal transfer ribbon while easily imparting the anti-static function to the ribbon.
Accordingly, the present invention has been achieved for solving the aforementioned problems, and an object of the present invention is to provide a thermal transfer ribbon having an anti-static function, being excellent in durability such as anti-abrasion property, alcohol resistance and image quality and having a good printing sensitivity.
This and other objects can be achieved according to the present invention by providing, in one aspect, a thermal transfer ribbon comprising:
a substrate; and
a color layer disposed on the substrate, the color layer comprising a binder resin and a color material,
wherein the color material comprises a first carbon black having a dibutyl phthalate (DBP) oil absorption of 50-150 ml/100 g and a BET specific surface area of 50-250 m2/g and a second carbon black having a DBP oil absorption of 350-500 ml/100 g and a BET specific surface area of 800-1300 m2/g, and the color layer contains the first and second carbon blacks at a total amount of 10-25 wt %.
In the thermal transfer ribbon described above, it is preferable that the thermal transfer ribbon further comprises a peeling layer, and the color layer is disposed on the substrate via the peeling layer.
Further, it is also preferable that the peeling layer comprises a wax.
In addition, it is also preferable that a ratio (a:b) of a weight (a) of the first carbon black to a weight (b) of the second carbon black is set within a range of 95:5-80:20.
Furthermore, it is also preferable that the color layer is formed by a solvent coating method using a coating liquid into which the binder resin is dissolved and the color material is dispersed.
Still furthermore, it is also preferable that an ethylene-vinyl acetate copolymer containing 19-28% of a vinyl acetate component is used as the binder resin of the color layer.
In addition, it is also preferable that the thermal transfer ribbon further comprises a peeling layer comprising a micro crystalline wax or carnauba wax, and the color layer is disposed on the substrate via the peeling layer.
Furthermore, it is also preferable that an adhesive layer containing no pigment is disposed on an outermost surface of the color layer.
In another aspect of the present invention, there is also provided a method of manufacturing a thermal transfer ribbon comprising the steps of:
providing a substrate;
providing a coating liquid prepared by dissolving a binder resin and dispersing a color material into a solvent, in which the color material comprises a first carbon black having a dibutyl phthalate (DBP) oil absorption of 50-150 ml/100 g and a BET specific surface area of 50-250 m2/g and a second carbon black having a DBP oil absorption of 350-500 ml/100 g and a BET specific surface area of 800-1300 m2/g, and the coating liquid contains the first and second carbon blacks at a total amount of 10-25 wt % in terms of solid content; and
coating the coating liquid onto the substrate so as to form a color layer.
In the method of manufacturing the thermal transfer ribbon described above, it is preferable that the substrate is further provided with a peeling layer, and the color layer is formed on the substrate via the peeling layer. Further, it is preferable that the peeling layer comprises a micro crystalline wax or carnauba wax.
In addition, it is also preferable that a ratio (a:b) of a weight (a) of the first carbon black to a weight (b) of the second carbon black is set within a range of 95:5-80:20.
Furthermore, it is also preferable that an ethylene-vinyl acetate copolymer containing 19-28% of a vinyl acetate component is used as the binder resin.
Furthermore, it is also preferable that an adhesive layer containing no pigment is further formed on an outermost surface of the color layer.
Functions of the present invention are as follows.
The thermal transfer ribbon of the present invention has a structure comprising a substrate and a color layer containing a binder resin and color material as essential components and formed on the substrate, in which the color layer contains the color material at an amount of 10-25 wt %, and the color material comprises at least one carbon black (referred to as xe2x80x9cfirst carbon blacksxe2x80x9d) having the DBP oil absorption of 50-150 ml/100 g and the BET specific surface area of 50-250 m2/g and at least one carbon black (referred to as xe2x80x9csecond carbon blackxe2x80x9d) having the DBP oil absorption of 350-500 ml/100 g and the BET specific surface area of 800-1300 m2/g. The first carbon black is excellent in dispersibility in solution while the second carbon black can easily form a grain structure obtainable a high electrical conductivity.
In the present invention, the above two kinds of the first and second carbon blacks are combined so as to reduce the total amount of carbon black, so that adequate anti-static property can be obtained even if the total amount of the carbon black is relatively small. As a result, there can be obtained a thermal transfer ribbon excellent in the uniformity of coated layer and the printing sensitivity or the like as well as the antistatic property.
Further, when the mixing ratio of the first and second carbon black is controlled so as to set a ratio of a weight of the first carbon black to a weight of the second carbon black within a range of 95:5-80:20, and/or when the binder resin for constituting the color layer is mainly formed of the ethylene-vinyl acetate copolymer (EVA) containing the vinyl acetate (VA) component at 19-28% and the color layer is formed by a solvent coating method using an organic solvent into which the EVA copolymer is dissolved, so that a coated layer having an improved uniformity can be obtained. As a result, there can be obtained a thermal transfer ribbon being excellent in the anti-static property, the durability such as the anti-abrasion property, the alcohol resistance and the like and having a good printing sensitivity, and being capable of forming an image with high quality.
As described above, in the thermal transfer ribbon of the present invention, two kinds of the first carbon black excellent in dispersibility in a solution and the second carbon black having a high electrical conductivity are combined, so that a sufficient anti-static property can be imparted to the ribbon even if the total amount of the carbon black contained in the color layer is small. Accordingly, the dispersibility of the carbon black in the coating liquid for color layer is not lowered, so that there can be provided a thermal transfer ribbon being excellent in the uniformity, the anti-abrasion property, the printing sensibility and being applicable to a high-speed printing type thermal transfer printer.